JP-2002-130042A shows a method of calculation of an estimated cylinder-intake-air amount, which is adopted in an open-loop air-fuel ratio control. The estimated cylinder-intake-air amount is calculated based on an output from an airflow sensor according to an intake-air-system model simulating a behavior of the intake air flowing from a throttle valve to a cylinder. However, when an error (a model error) of the estimated cylinder-intake-air amount is increased due to a dispersion in producing the system and a deterioration with age, the error is hardly compensated. Thus, when the method described above is adopted in the open-loop fuel ratio control, a robustness thereof may be deteriorated.
U.S. Pat. No. 5,384,707 shows a diagnostic method of an intake air amount sensor in which a diagnosis is conducted by comparing an intake air amount calculated based on an output of a throttle position sensor and an engine speed, an intake air amount detected by an airflow meter, and an intake air amount calculated based on an air-fuel ratio of exhaust gas detected by air-fuel ratio sensor and a fuel injection amount. The intake air amount calculated based on the throttle position and the engine speed is referred to as a throttle-base intake air amount hereinafter.
Dust in the intake air may adhere on a throttle valve. As the dust adhering on the throttle valve, which is referred to as a deposit, increases, the air passing through the throttle valve is decreased even if the throttle position is not changed, so that the calculating error of the throttle-base intake air amount increases.
Thus, in the system where the diagnosis for the intake air amount sensor is conducted based on the throttle-base intake air amount, it may erroneously diagnoses that the intake air amount sensor has malfunctions even though the intake air amount sensor is normal when the amount of deposit is increased.
When the diagnostic method described in U.S. Pat. No. 5,384,707 is applied to the system where the intake air amount is calculated based on the intake pipe pressure detected by the intake pipe pressure sensor in order to determine the fuel injection amount, the malfunction of the intake air pipe pressure sensor is conducted by comparing the intake air amount calculated based on the output from the intake air pipe pressure sensor, the throttle-base intake air amount, and the intake air amount calculated based the air-fuel ratio and the fuel injection amount. However, when the deposit on the throttle valve is increased, the normal intake pipe pressure sensor may be determined as the sensor having malfunctions due to the calculating error of the throttle-base intake air amount.